The present invention relates to a molded rubber material suitable for use as a golf ball material, particularly a spherical molded rubber material suitable as the core material in solid golf balls such as two-piece and three-piece golf balls. More specifically, the invention relates to a molded rubber material for golf balls which has a suitable hardness and resilience and an excellent durability. The invention additionally relates to a method of producing such a material, and to a golf ball in which such a material is used.
One-piece golf balls, and the solid cores which are enclosed, either directly or over an intervening mantle layer, by a cover in multi-piece golf balls, can be obtained by molding under applied heat and pressure a rubber composition made up of a rubber component such as polybutadiene as the base material and containing also an unsaturated carboxylic acid metal salt (e.g., zinc acrylate), dicumyl peroxide, and other ingredients. The unsaturated carboxylic acid metal salt serves primarily as a co-crosslinking agent or a crosslinking aid in the rubber composition, and has a large influence on the crosslink structure and crosslink density of the rubber. The unsaturated carboxylic acid metal salt is most preferably zinc methacrylate or zinc acrylate. This zinc methacrylate or zinc acrylate is generally incorporated in a large amount with respect to the rubber component such as polybutadiene. When the various ingredients are mixed and worked together to form the rubber, it is known that if the dispersion is not uniform, the molded material subsequently obtained will have diminished physical properties such as hardness, resilience and durability.
To resolve such problems, methods involving the use of zinc acrylate surface-coated with a higher fatty acid such as stearic acid or a metal salt thereof (JP-A 59-141961, JP-A 60-92781) have been disclosed. Other methods that have been disclosed include a method involving the use of zinc acrylate obtained by dispersing zinc oxide in an organic solvent within the presence of an anionic surfactant while concurrently reacting the zinc oxide with acrylic acid and a higher fatty acid in the organic solvent (JP-A 9-202747), and a method involving the use of zinc acrylate that has been dry-blended with solid rubber.
However, while the above methods all improve the dispersibility of zinc acrylate added to a rubber composition relative to the pre-existing art, there remains room for improvement in the hardness, resilience and durability of the spherical molded rubber material obtained with crosslinking.